Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dsones on January 22, 2026, 08:24:55 pm
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I just finished a 5E3 build and tested voltages with no tubes. Voltages were good. I plugged in the rectifier tube and the first 25uf 25v capacitor blew up. I can't see anything wrong. However, when I was checking components for continuity to ground I noticed the 6.3v heater pins have continuity to ground. I checked the lamp holder and the two lugs on the lamp holder have continuity to ground. Is that normal for a 5E3 circuit. Any ideas why the cap would blow up? I checked the orientation of the cap and that was correct. I tried attaching photos but got error message file was to large. The wiring diagram and schematic I used are the ones from Fender. Thanks
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You can resize your photos to make the file size smaller (how you do it depends on whether you are using Mac, PC, phone or whatever). What do you mean by first 25uf cap? Where exactly is it and what is it attached to?
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Power tube bypass cap connected to pin 8 of power tube.
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Pictures/schematics please.
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Did you wire your electrolytic cap reverse polarity? I attached a populated board before I soldered it. Note the positive polarity marking "+".
You're likely just reading the low resistance of the windings on the heater secondary taps. Not a short to ground. Put it on resistance. What do you read? Very low? Tenths of an ohm.
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I've added smaller sized photos.
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I just finished a 5E3 build and tested voltages with no tubes. Voltages were good. I plugged in the rectifier tube and the first 25uf 25v capacitor blew up. I can't see anything wrong. However, when I was checking components for continuity to ground I noticed the 6.3v heater pins have continuity to ground. I checked the lamp holder and the two lugs on the lamp holder have continuity to ground. Is that normal for a 5E3 circuit. Any ideas why the cap would blow up? I checked the orientation of the cap and that was correct. I tried attaching photos but got error message file was to large. The wiring diagram and schematic I used are the ones from Fender. Thanks
The 6.3V coils in the PT have very low resistance, commonly about 0.4 ohms. This indeed can result in a multimeter to show continuity. You said the voltages were good, does this include the 6.3 volts?
The cap blowing up could be due to a short ,or indeed wiring in reverse polarity such that it shorted. Less likely, could also be an old bad cap. You said "the first" but from which direction? Do you mean V1?
NEver mind you said 6V6 bypass cap... Thanks
BTW, your filament wiring is pretty random. Should be twisted pair to cancel hum, and run either against the chassis and out of the way of other wires or "flown" well above the tube sockets also away from other wires. And not wrapper around tube sockets. Running them separately though other wiring invites noise and oscillation problems. Refer to Merlin: https://www.valvewizard.co.uk/heater.html
(https://www.valvewizard.co.uk/heater10.jpg)
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Seems like the electrolytic caps are correct polarity. No idea why it would blow. You'd think the PT primary fuse would have blown. Just happened on my recent build of a different amp. I left a rectifier diode lead long while populating the board and forgot to clip it off before I mounted the board. Diode lead shorted the HT directly to the metal standoff/ground and blew the primary fuse. So thankful my PT didn't fry. That was a silly mistake.
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I just finished a 5E3 build and tested voltages with no tubes. Voltages were good. I plugged in the rectifier tube and the first 25uf 25v capacitor blew up.
I think your solution can be found in the thread you posted on The Amp Garage.
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Can't see where the red B+ wire from the rectifier connects under the board? Is it mistakenly connected to the cathode?